1. Field of the Invention
The present invention relates to an element substrate which is resistant to operation errors caused by a noise generated based on current fluctuation, capable of stable printing, and particularly suitable for an inkjet printhead, and a printhead, head cartridge, and printing apparatus using the element substrate.
2. Description of the Related Art
An inkjet printhead is conventionally known, which discharges ink from a plurality of discharge orifices using thermal energy. To obtain a stable discharge characteristic in the printhead, it is necessary to apply a stable voltage to heaters. A printhead element substrate has a plurality of heater arrays. When all heaters of a heater array are driven simultaneously, a large current flows to the ground wirings and the driving power supply wirings for supplying power to the heaters, and the voltage considerably drops due to the wiring resistance. If the voltage applied to the heaters varies because of the voltage drop, the ink discharge amount also varies, and a stable discharge characteristic is hard to obtain. To suppress voltage drop and obtain a stable discharge characteristic, a recent printhead element substrate limits the number of heaters to be driven simultaneously. More specifically, heaters are divided into a predetermined number of blocks and sequentially driven using so-called time-divisional driving, thereby applying a stable voltage to the heaters (Japanese Patent Publication Laid-Open No. 07-68761).
As described above, when a plurality of heaters are simultaneously driven, a large current flows to the driving power supply wirings and ground wirings. In this case, a noise generated based on current fluctuation generated by inductive coupling in the TAB wirings of the printhead poses a problem. The TAB wirings are provided on one side from the viewpoint of cost reduction and manufacturing ease of the printhead. Hence, the driving power supply wirings to apply the driving voltage to the heaters on the element substrate, the ground wirings, and logic signal wirings to send a signal to a logic circuit on the element substrate are formed in parallel. Hence, the noise generated by inductive coupling is superimposed on the logic signal. This may cause operation errors of the logic circuit provided on the element substrate. To prevent this, the element substrate using time-divisional driving delays the timings of driving pulses to be applied to heaters in a selected block in the order of nsec. The current flow per unit time is reduced in this way, thereby preventing the noise generation and operation errors of the logic circuit on the element substrate.
In recent inkjet printing apparatuses, discharged ink droplets have increasingly become small for high-quality image formation. Along with improvement of image quality, the printing speed is also required to be higher. However, it is difficult to implement high-speed printing if the discharge ink droplets are small. For example, if the ink discharge amount simply decreases to ½, the number of times of ink discharge must double. Hence, the printing speed decreases to ½.
To prevent the decrease in printing speed caused by small ink droplets, it is necessary to apply the same amount of ink to a print medium in per unit time as before. The decrease in printing speed can be prevented by increasing the number of heaters arranged on the element substrate. However, if only the number of heaters is simply increased without changing their pitch, the element substrate becomes large, and the printhead incorporating the element substrate becomes bulky. The printhead scans in the inkjet printing apparatus at a high speed. Hence, a bulky printhead generates vibration and noise. A bulky printhead also increases cost. To increase the number of heaters without changing the size of the element substrate, a method for increasing the heater arrangement density has been proposed.
When the arrangement density of heaters rises, the number of heaters to be driven simultaneously also increases. When the number of heaters to be driven simultaneously also increases, the current flow per unit time to the driving power supply wirings further increases. For this reason, the conventional delay method using time-divisional driving can hardly suppress a noise generated based on current fluctuation generated by inductive coupling in the TAB wirings of the printhead.